I. Field of the Invention
The present invention relates generally to an apparatus for balancing a rotating body and, more particularly, to such an apparatus which utilizes a laser beam to ablate compensating material from the rotating body to achieve body balance.
II. Description of the Prior Art
High speed rotating bodies, such as turbine wheels, require balancing of the rotating body prior to its installation. For example, an unbalanced turbine wheel will result in bending stress caused by the large rotational inertia force which not only subjects the bearings to excessive wear and tear, but also may result in turbine wheel failure.
Heretofore, high speed rotating bodies, such as turbine wheels, have been balanced at low speeds on semi-automated machinery. Balance sensors on the machinery sense the degree of unbalance of the turbine wheel, and computing means determine the amount and location of compensating material to be removed from the wheel in order to effect the proper wheel balance. Previously the compensating material has been removed manually, such as by a hand drill, and, for this reason, it is necessary to stop the rotation of the wheel prior to the removal of the compensating material. Hand removal of the compensating material is disadvantageous in that such removal is not only sensitive to operator technique but also unavoidable errors inherent in any manual operation result in not only the removal of an improper amount of compensating material but also removal from an incorrect location. Moreover in practice it has been found that low speed balancing of turbine wheels does not result in a sufficiently fine balancing to permit high speed operation of the rotating body.
In view of the difficulties and disadvantages in the hand removal of compensating material, it has been suggested that a high power laser be utilized to ablate compensating material from the rotating body. It has, for example, been previously suggested to actuate or fire a laser at a pre-chosen balance plane, such as a conventional balance ring, for a very short duration in order to remove compensating material from the rotating body.
In practice, however, this laser machining method has been found to be inadequate. One problem is that in order to remove compensating material from a given angular location on the turbine balance ring, the laser can be fired for a short duration only once per revolution of the rotating body in order to ablate the same location on the balance ring. Since the laser beam ablates only a relatively small amount of compensating material per turbine wheel revolution, such laser balancing systems have proven unduly time consuming in operation.
The previously known laser balancing systems suffer the further disadvantage in that as the rotational speed of the rotating body is increased to its expected operational speed, often times exceeding 18,000 RPM, the area of ablation by the laser becomes circumferentially oblong due to the high rotational speed of the body. While shorter laser pulses may reduce this circumferential oblongation, very short laser pulses are not only difficult to obtain with precision, but also such short pulses reduce the amount of compensating material ablated from the rotating body per laser pulse.